Electro-slag remelting (ESR) is a secondary refining process for metals. ESR is used for further purification of the metal after completion of the primary extraction and refining operations. ESR generally uses a raw material in the form of a solid consumable electrode which is either cast, wrought or composed of scrap. A layer of slag material contained in a cooled mould is resistance heated and melted by an electric current flowing between the electrode and a cooled base plate within the mould. As the temperature of the slag layer rises above the melting point of the metal, droplets melt off the tip of the electrode, fall through the slag and collect in a pool on the base plate. The pool of molten metal is cooled by the base plate and the mould walls and solidifies to form an ingot. The electrode is fed into the slag layer with the solidified ingot, which now acts as a secondary electrode, being progressively built up through further cooling.
Refinement within an ESR operation takes place because of a reaction between the metal and the slag during formation of droplets on the electrode tip. The detached droplets fall through the slag and collect in a pool at the top of the ingot. In most ESR operations it is preferred that the pool of molten metal be shallow. Such a condition produces a sound ingot that is free of porosity and pipe, as well as a high yield. An increase in current through the electrode typically increases the pool depth. An inadequate melt rate of the electrode produces an ingot having a rough, corrugated surface which results in a poor yield. A melt rate which is too high results in a large pool depth and increases porosity and pipe.
The ESR operation shown in U.S. Pat. No. 3,868,987 which is directed to an apparatus in which a liquid inert gas (e.g., argon or nitrogen) is placed on top of the molten slag. The liquified inert gas may be fed into the ingot mold by means of a single nozzle or by means of multiple nozzles via a manifold. The object of this structure is to isolate the slag and the electrode from the ambient atmosphere. Ambient air may act on the slag, introducing elements such as oxygen or hydrogen, which are then transferred by the slag to the purified metal. The layer of gas which is particularly inert with respect to the metal may be utilized to protect the slag from the ambient air and to a certain extent that which surmounts the slag.
U.S. Pat. No. 3,989,091 is directed to a method for electro-slag remelting of titanium or titanium alloys. This patent also discloses supplying a blanket of inert gas to the surface of the molten slag as well as supplying inert gas to the interface between the molten slag and the molten metal at the top of the ingot mould.
U.S. Pat. No. 4,120,695 is directed to an apparatus for controlling the melting .rate of a plurality of consumable electrodes which are simultaneously remelted. Because individual consumable electrodes may melt at different rates, a cooling gas is blown from a nozzle adjacent the electrode which is melting too quickly and, therefore, is immersed into the slag to a depth lesser than that of the other electrode. By cooling the electrode, its melting rate decreases and hence it becomes immersed more deeply into the slag, diminishing any unbalance in the melting of the electrodes.